1. Field of the Invention
The present invention relates to a voltage stabilizer apparatus for use with alternating current (AC) power sources. In particular, the present invention relates to an AC voltage stabilizer with digitally-controlled emulation of servotransformer operation using tap-switching technology.
2. Discussion of Background
AC power lines are subject to a number of different types of voltage disturbances. These include spikes (i.e., pulses of very high voltage and current), which are most commonly caused by lightning and usually last less than a second; surges and sags, which are periods of less severe, too-high or too-low voltage lasting from several seconds to a few minutes, usually caused by faulty power company switching or by other devices on the line; brownouts, which are longer sags lasting from several minutes to several hours; and blackouts, which are periods of near-zero voltage, which can be caused by blown fuses or tripped circuit breakers after a lightning strike or other line problem.
Users of computers and other electronic equipment are familiar with the problems caused by power-line spikes and surges. So-called xe2x80x9csurge-suppressingxe2x80x9d (more correctly, xe2x80x9cspike-suppressingxe2x80x9d) power strips or adapters are now widely available. However, these devices are of little value in protecting against sustained power-line surges and of none at all against sags or brownouts. In areas where severe sags, surges or brownouts occur frequently, voltage-sensitive devices such as motors, computers and other electronic equipment, and even ordinary light bulbs have a limited life expectancy. These problems are ubiquitous in the historically less-developed countries, and even in remote areas of the developed countries where generators or long stretches of low-tension cable may be needed to provide electrical power. In many countries, the infrastructure needed to support widespread use of these technologies is inadequate. In particular, the lack of steady, dependable electrical power has proved to be an important factor in limiting the potential market for electrically-powered consumer goods (refrigerators, air conditioners, television sets, personal computers, etc.).
Many methods and devices are available for stabilizing local line voltage, but typically represent difficult compromises, playing off smoothness and accuracy of control against cost. Most commercially-available voltage stabilizers rely on transformers whose step-up or step-down ratios can be changed to help compensate for input-voltage changes. These devices fall into two broad classes: discrete tap switchers or xe2x80x9crelay boxes,xe2x80x9d and servo-transformers.
Tap switchers are simple and inexpensive, but do not provide smooth voltage regulation. In such a device, one or more relays select various taps of a transformer (typically an autotransformer) so that the output voltage is raised or lowered in steps to help compensate for changes in the input voltage. This principle is illustrated in FIG. 1A for a nominal 230-volt line using a single relay, where an autotransformer 10 is provided with three taps 12, 14 and 16, arranged so that an AC voltage applied between taps 12 and 14 results in a higher (xe2x80x9cstepped-upxe2x80x9d) voltage appearing between taps 14 and 16. (For clarity, transformer 10, an iron-cored AC power transformer, is shown as a simple coil or series of windings in FIG. 1A and the following Figures.) The selected step-up ratio depends upon the particular application, but is typically within the range of about 10-30%.
In an unbalanced circuit, the AC (alternating current) hot line is normally connected to tap 12 through a terminal 12a, and the neutral line to tap 14 through a terminal 14a. A relay 20 connects either the input voltage at tap 12 or the stepped-up voltage at tap 16 to a terminal 22. Relay 20 is driven by a control circuit (represented schematically as 24) which selects tap 12 when the input voltage is generally above a threshold 26, or tap 16 when the input voltage is generally below this threshold. By xe2x80x9cgenerallyxe2x80x9d it is meant that the control is not exact; hysteresis effects are usually present, and indeed are desirable to prevent relay chatter and prolong the life of relay 20. The output is then taken between terminal 22, which is the new AC hot line, and neutral output terminal 14b. Threshold 26 is chosen so that the output voltage remains in the vicinity of a selected voltage represented by a line 36 (FIG. 1B), over a wider range of input voltages than if no compensation were made.
The result is shown graphically by line segments 30, 32 and 34 (FIG. 1B). When the input voltage is below threshold 26, and neglecting the effects of loading, relay 20 selects tap 16. The output voltage measured between taps 14 and 16 is higher than the input voltage by the step-up factor of transformer 10, typically between about 10-30%, as shown by line segment 30. As the input voltage rises above threshold 26, control circuit 24 causes relay 20 to change position, selecting tap 12 instead of tap 16, and the output voltage changes abruptly as shown by line 32. At still higher input voltages, the output equals the input as shown by line segment 34.
If the input voltage then drops below threshold 26, relay 20 again changes position to select tap 16 and the output voltage is again stepped up to compensate for the input voltage drop. For comparison, line segment 38 represents the output voltage if no compensation is made.
When transformer 10 is supplying current to a load, the graph shown in FIG. 1B is not completely accurate since some voltage drop occurs within the transformer, and the output voltage is correspondingly lower. Hence, selecting the transformer tap according to the output voltage, rather than the input, would in theory provide superior control. In commercial tap switchers, however, this is not often done because it can cause oscillation between positions, so that the xe2x80x9cstabilizerxe2x80x9d actually makes the output voltage less rather than more stable.
The quality of voltage regulation available from a tap switcher can be improved by adding more steps, so that the intervals between the steps can be made smaller and/or so that a wider span of input voltages can be handled. More steps, however, require correspondingly more relays and transformer taps, resulting in greater circuit complexity and higher cost. These types of devices normally operate in simple xe2x80x9cdaisy-chainxe2x80x9d fashion, so that a separate tap and relay are required for each step.
For example, FIG. 2A illustrates a tap switcher using an autotransformer 10 with four taps (not separately labeled) and three relays 20a, 20b, and 20c acting successively at three different thresholds 26a, 26b, and 26c, respectively, under the control of circuit 24. Such a stabilizer might, for example, provide either straight-through operation as shown by line segment 34 (FIG. 2B), boosts of 20% or 40% for line undervoltages as shown by line segments 30a and 30b, respectively, or 20% bucking (voltage decrease) for line overvoltages as shown by line segment 30c. Line segments 38a and 38b represent the output voltage if no compensation were made.
As in the circuit of FIG. 1A, abrupt voltage steps 32a, 32b and 32c occur each time a relay switches. However, a larger number of steps permits either stabilization of the output voltage near a desired level 36 over a wider input voltage range, smaller individual steps so that the output voltage remains closer to level 36 over this range, or both.
Some products designed mostly for audiophile or recording-studio use perform a similar function to the circuits of FIGS. 1A and 2A using solid-state switching devices rather than electromechanical relays. This approach speeds response and minimizes switching noise, but tends to be quite costly because of the added complexity of the circuitry.
Servo-transformers provide much better and smoother, nearly stepless regulation than tap switchers, but at the cost of greater mechanical complexity, higher price, and relatively slow response. Such a device and its voltage response are shown in FIGS. 3A and 3B. Instead of multiple discrete taps, autotransformer 10 is typically equipped with only two taps 12 and 14 plus a sliding brush 50 which is able to contact any selected one of the transformer windings. This is usually accomplished by winding transformer 10 with heavy copper wire on a ring-shaped armature, then grinding one end surface flat (including all windings) so that the flat surfaces of successive windings lie in a common ring-shaped plane. The ground surfaces of the windings are usually plated with a precious metal to prevent tarnishing. The brush, which is usually made of graphite, is mounted on a pivoting arm so that, as the arm rotates, the brush contacts each winding in turn.
As brush 50 moves along the windings of transformer 10, and with an AC voltage applied between taps 12 and 14, the step-up or step-down ratio depends upon the position of brush 50. This ratio changes as brush 50 is moved from one winding to another. Typically, one tap 14 is located at an end of the winding and serves as a common neutral, while the other tap 12 is located partway along the winding and serves as the hot input. Brush 50 is connected to output terminal 22. As a result, the ratio of the output to input voltages can be changed from near zero to well above unity in a series of very small steps: one step for each turn of winding 10 which is exposed along the path of brush 50.
A mechanically-variable autotransformer such as this, without other components, is sometimes called a xe2x80x9cvariac.xe2x80x9d Because of its mechanical complexity and the grinding and plating operations required in its manufacture, a variac is typically more expensive, as well as significantly larger and heavier, than a multitapped autotransformer with the same overall power rating.
A variac can be made into a voltage stabilizer by adding a servomotor (represented schematically as 52) which moves brush 50 back and forth along the windings of transformer 10 under the control of a circuit 24. Linkage between servomotor 52 and brush 50 is usually through a gearbox or other suitable type of speed reducer 54.
Since the voltage steps are small, feedback control can be used to minimize output-voltage changes as the load varies. Typically, two threshold voltages 60a and 60b are set up, 60a above and 60b below a desired output voltage 36, and spaced further apart than the maximum voltage step which occurs on the motion of brush 50 from one winding of transformer 10 to the next adjacent winding. The output voltage is monitored, typically by being rectified, filtered and compared with DC (direct current) references corresponding to these thresholds. Should the output voltage move above level 60a, brush 50 is moved to contact a lower-voltage turn of winding 10. Conversely, should the output voltage move above below 60b, brush 50 is moved to contact a higher-voltage turn. The output voltage is continually stabilized in this way.
On a rising input voltage, the output voltage repeatedly rises to threshold 60a and is corrected downward in small steps throughout the range over which stabilization is possible, as generally indicated by line 62 (FIG. 3B). If the input voltage falls again, the output voltage will repeatedly decrease to threshold 60b and be corrected upward in small steps, as generally indicated by line 64. (For purposes of illustration, thresholds 60a and 60b are shown relatively further apart, and the voltage correction steps in lines 62 and 64 correspondingly larger, than would be typical in a real stabilizer. As in FIG. 1B, line 34 represents the output voltage if no correction were being performed.)
Since a variac can provide voltage step-up or step-down ratios ranging down almost to zero but upward only to some set ratio above unity, the range over which it is possible to keep the output near desired output level 36 ranges from a minimum voltage 66 set by the maximum step-up ratio, upward to a limit 68 determined by the ability of winding 10 to withstand overvoltage. With an input voltage less than voltage 66, the output voltage equals the input voltage multiplied by the maximum step-up ratio as indicated by a line 70. Operation at an input voltage greater than limit 68 should be avoided, since equipment damage could occur.
Because of the need for mechanical motion, correction for input-voltage or load changes in the circuit of FIG. 3A is typically rather slow. If control circuit 24 is improperly designed, moreover, or if thresholds 60a and 60b are set too close together, brush 50 may move too far before circuit 24 can respond. In such a case, oscillation takes place, the output to the load becomes unstable, and brush 50 may quickly wear out.
The advantage of a servo-transformer voltage stabilizer such as that shown in FIG. 3A is that, since it provides a large number of very small steps, voltage regulation can either be very accurate, be performed over a very wide range of input voltages, or both. Because of its mechanical complexity, however, such a stabilizer is larger, heavier, and more costly than a tap switcher of equal power-handling capacity. Because of the large number of moving parts it contains, it is also more liable to failure and requires much more frequent maintenance.
Response approximating that of a servo-transformer can be achieved using conventional tap-switching technology if a very large number of taps and relays are used. However, because of the expense of so many relays and the complexity of the circuitry needed to drive them, and because the geometry of a conventional power transformer limits the number of taps which can be placed on its windings to a dozen or so, this is not achievable at reasonable cost.
Additional types of voltage stabilizers use devices such as ferroresonant transformers. These provide much smoother voltage regulation and have no moving parts, but are cumbersome, relatively costly, and often generate a continuing audible hum that is annoying to many listeners.
There is a need for a voltage stabilizer apparatus that provides smooth, noise-free regulation in a relatively small, lightweight package, at low cost and with high power efficiency.
According to its major aspects and broadly stated, the present invention is a voltage stabilizer apparatus that uses cost-effective, digitally-controlled emulation of servotransformer operation to provide smooth voltage control. The invention makes use of tap-switching on both the primary and the secondary sides of a transformer, thereby allowing a relatively small number of relays to provide a relatively large number of voltage step-up and step-down ratios.
An important feature of the present invention is the use of tap-switching technology to emulate servotransformer operation, that is, to xe2x80x9cservoxe2x80x9d a voltage up or down under control of a feedback loop. In a conventional tap switching circuit, the number of step-up and step-down ratios is equal to the number of relays used to perform the switching, that is, one less than the number of taps. By switching on both the primary and secondary sides of an autotransformer, the maximum number of achievable ratios is increased to Nmax=(m*nxe2x88x92p+1) where m is the number of primary taps, n the number of secondary taps, and p the number of taps which are common to both sets. For m=n=p=5, for example, there are 21 possible ratios, more than four times as many as for a conventional tap switcher with 5 relays. For m=n=p=10, the invention offers 91 possible ratios instead of the 9 afforded by a conventional device.
Another feature of the present invention is the spacing of the taps. For an approximately evenly spaced series of step-up and step-down ratios, the taps are placed at logarithmic (or approximately logarithmic) intervals along the transformer winding. Two overlapping sets of such taps can be used: one set placed at a basic narrow spacing ratio Rn and the other at a wider spacing ratio Rw, so that combinations of the two sets form ratios at approximately equal logarithmic spacing. Using one set of taps as the inputs and the other set as the outputs then gives the desired succession of ratios.
Still another feature of the present invention is the ability to add additional components to the apparatus, including but not limited to devices for sensing under- or over-voltages, devices for preventing unwanted cycling under load, rectifiers, arc-suppression devices, and circuitry for interfacing with the user""s electronic equipment. If desired, LEDs or other indicators may be included for providing feedback on the operational state of the voltage stabilizer apparatus.
Other features and advantages of the present invention will be apparent to those skilled in the art from a careful reading of the Detailed Description of Preferred Embodiments presented below and accompanied by the drawings.